As is well known in the art, water systems contain ingredients, either naturally occurring, as contaminants, or formed by the combination of anions and cations, which can and often do cause deposition problems.
For example, depending on the water source and process conditions, industrial water can contain alkaline earth metal or transition metal cations such as calcium, barium, magnesium, iron, etc. and such anions as carbonate, phosphate, sulphate, oxalate, silicate, etc. The combination of these anions and cations could, accordingly, form such potential depositing salts as calcium carbonate, calcium sulphate, calcium phosphate, magnesium carbonate, magnesium sulphate, etc. When the concentration of any of such salts which are formed exceeds their solubility limit, they precipitate out of the water in the form of scale. The concentration of these scale forming salts can increase as a result of partial water evaporation, or changes in pH, temperature or pressure of the water. The amount of scale formation generally depends on pH, temperature and type of salt formed. The scale thus formed will deposit on surfaces in contact with the aqueous medium, such as flow pipes, storage tanks, heat exchanger surfaces, etc. These deposits can prevent effective heat transfer, interfere with fluid flow through pipes, facilitate corrosion, and harbor bacteria.
Deposit control agents, such as phosphates, phosphonates, and polyacrylates, show similar responses as the concentration of calcium is increased in cooling waters and the like with the potential for precipitation of slightly soluble calcium salts. At very low (substoichiometric) treatment levels, these deposit control materials inhibit the nucleation and growth of crystals of calcium salts. The mechanism for this activity involves adsorption of the deposit control agent at the active growth site of the forming microcrystallites. If the concentration of calcium is increased, turbidity develops in the cooling water, indicating the formation of insoluble, calcium-deposit control agent adducts. If the deposit control agent concentration is increased to stoichiometric concentrations, this turbidity can be removed by chelation of the calcium ion to produce soluble calcium-containing species.
Because of the economics of water treatment in cooling systems, deposit control agents must function at substoichiometric concentrations. In waters containing high calcium concentrations, such as might be found in cooling systems operating at high cycles of concentration, calcium tolerant deposit control agents offer a distinct advantage. The concentration of these materials can be increased to meet the deposit control demands of the system without concern for their removal by formation of calcium containing adducts.
Formation of calcium-deposit control agent adducts has obvious negative consequences. The active or "free" deposit control agent concentration is limited, thus limiting deposition and corrosion control. Also, the adduct itself may foul the cooling system through the formation of an adduct deposit.
To alleviate this problem, the calcium concentration is often controlled by operating at lower cycles of concentration. However, such procedure also has obvious economic disadvantages.
Thus, a deposit control agent that is tolerant to high calcium concentrations provides definite advantages when used in cooling water systems and the like. The high treatment concentrations that may be required due to the deposition potential created by high calcium concentrations can be used without fear of fouling or loss of corrosion protection. Cycles of concentration need not be limited, providing economic benefits and conservation of water.
Accordingly, there is a need in the art for a method of controlling deposition in high calcium ion content waters, which method does not result in the substantial formation of adducts comprised of calcium ions and the deposit control agent.
Most of the present-day corrosion inhibitor treatments comprise a phosphate and/or phosphonic acid constituent. Phosphate may also be contained within the makeup water, e.g., tertiary sewage treatment effluent. The reversion of the polyphosphates and the organic phosphates plus the use of alkaline operating conditions leads to the formation and deposition of highly insoluble calcium phosphate. Accordingly, there is a need in the art for a deposit control treatment which inhibits the formation of calcium phosphate deposits.